MASVINGO, Zimbabwe — The first lady of Zimbabwe’s display of power was unspoken, though clear, during the governing party’s annual congress, as she focused her speech on new party regalia featuring a teacup-shaped image of her country.

“We all drink from the teacup,” Grace Mugabe, the first lady, said, explaining that she had designed the regalia herself.

Not surprisingly, the next morning in Masvingo, the small town in southern Zimbabwe where the congress of the Zimbabwe African National Union-Patriotic Front, President Robert Mugabe’s party, was held recently, nearly all officials wore clothes adorned with Ms. Mugabe’s teacup design.

Ms. Mugabe — known mostly for her lavish overseas shopping trips until she entered politics just two years ago — has emerged as one of the main actors in the fierce maneuvering to succeed Mr. Mugabe that has engulfed Zimbabwe in the last year, as the president’s visible decline presages the end of an era.

She is, to many people, the real power behind the throne, vowing to keep her husband in office until his death while she consolidates her support. She told supporters recently that she was “already the president,” planning and doing everything with her husband.

The signs of Ms. Mugabe’s growing stature are unmistakable. On stage at the party congress, she sat closest to her husband, who, a couple of months shy of 93 years old, dozed through most speeches. Party leaders invariably praised her also — “Forward with President Mugabe! Forward with Dr. Amai Grace Mugabe!” — before others officially higher in the party hierarchy. A choir that usually sings the president’s praises composed a song for the first lady for the first time.1

“Be pleased to follow Mrs. Mugabe, a mother who has love, mother of the nation, the one who takes care of orphans,” the Mbare Chimurenga Choir sings in its new song, “Following Mother Mugabe.”

Though visibly asleep during most of the congress, Mr. Mugabe, the world’s oldest head of state and the only leader Zimbabwe has known since its independence in 1980, was selected as his party’s candidate in the 2018 presidential election. He would be 94 by then and, should he win, 99 by the end of his term.

At the congress, Mr. Mugabe appeared increasingly dependent on his wife, who is 51. When a waiter carrying bags of potato chips on a silver tray startled Mr. Mugabe, the first lady chose a bag, from which the president then slowly picked out one single chip after another. At a tree-planting ceremony, a seemingly confused president kept tapping a mound of dirt with his shovel until the first lady intervened — “honey,” she called him — by grabbing the shovel herself.

Whether the first lady’s power will survive her husband’s death is unclear. She is reported to head one of the two competing factions inside ZANU-PF, but is she its leader, or just a useful puppet for veteran survivors of Zimbabwean politics? After her husband dies, will she hop on a plane for Dubai or elsewhere in Asia, where she and her children have established homes? The Mugabes are thought to have more than $1 billion invested outside Zimbabwe, according to an American diplomatic cable released by WikiLeaks.

But if she succeeds in grabbing power, it would most likely be a continuation of her husband’s government. Changes critical to reviving Zimbabwe’s crumpled economy, including land reform, are thought politically impossible under Mr. Mugabe and would remain so under Ms. Mugabe, whose legitimacy derives from her husband’s legacy. Her elevation could also intensify tensions in Zimbabwe’s small political class by upsetting Mr. Mugabe’s lieutenants, many of whom have been waiting decades to take over.

Confident of their grip on power, the Mugabes flew out of Zimbabwe a few days after the end of the congress in mid-December for their annual extended holiday in Asia, where Mr. Mugabe is thought to have received medical care in Singapore and Malaysia, and where the first family owns real estate in Hong Kong.

Ms. Mugabe left for her latest holiday even though she was embroiled in a dispute with a Lebanese diamond dealer over a $1.35 million ring. According to a court document, Ms. Mugabe ordered the diamond from the dealer; issued the payment from a bank in Harare, the Zimbabwean capital; then canceled her purchase after the diamond had already been prepared. Ms. Mugabe demanded that the dealer refund the money to a bank account in Dubai, according to the court document.

The dealer, Jamal Ahmed, said he had refused to transfer the money to Dubai because it would be considered money laundering, but agreed to reimburse the first lady in installments, according to an affidavit submitted to the High Court of Zimbabwe. Men associated with Ms. Mugabe and her son from a previous marriage subsequently seized and occupied three of the dealer’s properties in Harare.

Wilson Manase, Ms. Mugabe’s lawyer, did not return calls and messages to his cellphone.

Ms. Mugabe had picked the diamond as her husband’s 20th wedding anniversary present to her.

The president and Ms. Mugabe became involved when she worked as a typist in the president’s secretarial pool. The president’s first wife — Sally, a much-loved figure in Zimbabwe although she was from Ghana — was terminally ill at the time and approved of the affair, Mr. Mugabe has said in the past.

To some who have long known Mr. Mugabe, the marriage to Grace changed the president’s priorities.

“Mugabe did change automatically — it was so dramatic,” said Margaret Dongo, a former ZANU-PF official now in the opposition, adding that Mr. Mugabe had never shown interest in money before his second marriage. “But Grace knew that it was the time to make money. Whatever she did, she made sure she benefited more out of it.”

Ms. Mugabe has run a large dairy business from a farm she owns and seized former white-owned farms from top ZANU-PF officials for her own family. According to American diplomatic cables released by WikiLeaks, Ms. Mugabe was engaged in the illegal mining of diamonds in eastern Zimbabwe. She was also involved in many commercial and residential construction projects, choosing to contract South Korean construction firms.

In 2007, in an assessment of Ms. Mugabe, the political officer in the American Embassy in Harare wrote that “Grace’s primary personal interest appears to be shopping.”

“We believe Grace has little or no political influence over her husband,” the officer wrote.

But two years later, Gunnar Foreland, a Norwegian ambassador with experience in Africa, warned his American counterpart about underestimating Ms. Mugabe’s influence over her husband. “She acts as a kind of gatekeeper, often controlling who sees him, and what information gets to him,” the ambassador said.

Ms. Mugabe formally entered politics in 2014, becoming leader of ZANU-PF’s women’s league. She was awarded a doctorate after only two months at the University of Zimbabwe. Supporters have engaged in the kind of mythmaking that had been reserved for the president, who is often described as walking on water and in other Christlike terms.

Namatirai Chivhanga, a top women’s league official, said the first lady’s loving embrace of the country’s orphans was so moving that one couldn’t help crying.

“Somehow she’s putting on Pampers and all the children are calling her Mama, Mama,” Ms. Chivhanga, a delegate to the congress, said, adding that she had seen Ms. Mugabe personally change the diapers of many orphans. “It’s amazing.”

Ms. Mugabe has also systematically secured support by using her wealth or access to state funds.

“She gave us thousands and thousands of chickens in all the provinces,” said Angeline Muchemeyi, the chairwoman of the women’s league in the Mashonaland West Province. “And the new cars — we got Ford Rangers. She’s good, she’s very good to us, that lady.”

Ms. Mugabe has treated potential rivals without mercy. She expelled from ZANU-PF a vice president and war hero, Joice Mujuru, by accusing her of engaging in treason, practicing witchcraft and wearing short skirts. She also initiated a fierce attack against another vice president and leader of a rival faction, Emmerson Mnangagwa.

Supporters of Mr. Mnangagwa said Ms. Mugabe was being used by more experienced politicians to undermine their candidate.

“She’s soft and inexperienced,” said Douglas Mahiya, spokesman of the National Liberation War Veterans Association, which supports Mr. Mnangagwa. “Even if she becomes president, it will be easier for these people to push her aside and take over.”

Ms. Dongo, the former ZANU-PF official, said the first lady’s political rise was part of the president’s plan to build a family dynasty. “The kids are too young and don’t have the capacity,” she said.

None of the Mugabes’ three children — the oldest is 28 — have shown any interest in politics. Mr. Mugabe has spoken of his disappointment in his two sons’ poor academic performance. The older son, Robert Jr., who is studying in Dubai, often posts photos on social media of himself partying with different women.